The present invention relates to nonreciprocal devices, such as isolators and circulators, operating at relatively low microwave frequencies (approximately 50 MHz–1 GHz), in relatively high power systems (approx. 50 Watts and higher) and under broadband conditions. More specifically, the invention relates to the center conductor circuit (further referred to as a circuit) having a central junction and arms for lumped element circulator/isolator with three ports and method for forming a resonant structure when this circuit is assembled with a ferrite and an isolating material. Externally applied biasing magnetic field allows the resonant structure to act as either circulator or isolator (when one of the ports is terminated).
The circuits for lumped element devices are well-known (see, for example, U.S. Pat. No. 5,838,209). In the known circuits, three arms extended from a central junction and folded over a ferrite are disposed on the ferrite surface in such a way that they intersect each other at specified angles in electrically isolated conditions and a DC bias magnetic field is applied to the ferrite. The arms in the known circuits may consist of one or more strips that are parallel to each other.
Circulators are usually connected to 50 Ohm transmission lines while isolators have one port connected to a matched 50 Ohm termination. In order to minimize the reflection of RF power, each arm, when incorporated into the resonant structure, should have 50 Ohm impedance, which requires the entire system to be symmetrical. In terms of attaining symmetry, the most crucial features in resonant structure are shape of the arms and geometry of strip intersection. In the narrow-band devices, a small asymmetry and difference in the arm impedances are tolerable since they can be compensated by a conventional tuning. In some particular applications the arms can also be differently shaped to obtain the required performance at narrow band (see, for example, U.S. Pat. No. 6,614,324). The broadband operation, however, is realized on the condition of well maintained symmetry between the arms. To achieve the symmetry, the overlapping pattern at strip intersections and disposition of all strips with respect to the ferrite(s) should be identical, or, at least, have minimal differences.
The simplest circuit is one which consists of the arms each having only one strip. In such circuit all three folded arms/strips intersect simultaneously in the central area of a ferrite forming one triple crossover. To ensure the electrical isolation at intersection, the strips are sandwiched between the layers of low loss thin insulation, such as dielectric films of Maylar, Kapton, and alike. The major drawback of this design is that at triple crossover the distance between the ferrite and strips varies from one strip to another. It means that the electromagnetic coupling between the strips and the ferrite varies from one strip to another as well, and because of that, the electrical characteristics of resonant structure become asymmetrical. Also, in a triple crossover area, the strip situated in a middle of the crossover becomes obstructed from the ferrite by two adjacent strips. This situation also contributes to the asymmetry. Another shortcoming of a single strip design is that the RF field associated with the strips is not uniform, but concentrated predominantly in the ferrite central area where the strips meet each other. To diminish the asymmetry and produce more uniform RF field the conventional design utilizes the circuits having more than one strip (typically two) at each arm. Two strips per arm being folded produce four intersections in each individual strip and twelve intersections in total. Conventionally, the resonant structure is formed by folding all strips of the same arm simultaneously. Because of multiple crossovers, the field distribution over the ferrite improves; however, triple intersections can stay. Therefore, device in operation becomes asymmetrical for the same reason as it was described for a single strip structure. The asymmetry can be reduced by folding the strips separately (weaving). This procedure reduces asymmetry by inverting the disposition of the strips at each consecutive intersection. But it also slows dawn the production process as the number of strips to be weaved increases.
With the increase of RF power passing through a device, avoiding a local overheating becomes more important. This can be done by distributing the RF field more evenly over the entire ferrite area. The uniformity of RF field can be achieved by increasing the number of strips in the arms. Increasing the number and width of the strips improves also the matching and broadband performance as the frequency decreases below 1 GHz.
Increasing the number of strips while using conventional folding methods leads also to the development of plurality of triple-intersection areas. For better performance, it is desirable to avoid all triple crossover areas at intersections, as they directly contribute to the development of asymmetry. If, for the symmetry reason, conventionally designed strips are folded and isolated separately, a dramatic increase of the layers of sandwich structure develops as number of strips in arms increases. With the increase of the number of layers and overall thickness some strips become positioned farther from the ferrite and, consecutively, become less coupled with resonant structure, thus decreasing the bandwidth and degrading the nonreciprocal action of circulator/isolator. As seen from above, the best high-power and broadband performance of lumped-element circulators and isolators can be achieved only if the increase in number of strips does not spoil the symmetry of arms and degrade the coupling between strips and the ferrite.
Accordingly, what is needed is a thin symmetrical multi-strip resonant structure for lumped element device wherein only two strips overlap at each intersection point, and a method for providing such structure. Such resonant structure, when used in nonreciprocal devices like circulator/isolator, has to provide adequate heat dissipation in relatively high power operation that is big enough electromagnetic field distribution area. Also, such resonant structure has to adequately operate within broadband conditions and at the frequency range less than 1 GHz.